1. Technical Field
The present invention relates generally to electrical contacts and probes for interconnecting integrated circuit (IC) devices and electronic components and systems, and specifically, to miniature contacts and probes having a low inductance and a low resistance. Applications include connectors for connecting electronic devices to a next level electronic hardware, test sockets, battery and charging contacts, and other applications requiring low contact inductance and low contact resistance. The connectors can be used in an interposer configuration to separably connect two oppositely disposed components having corresponding input/output (I/O) terminal arrays. Alternatively, the contacts can be permanently attached to a printed circuit board (PCB) or a system motherboard and provide a separable interface for an electronic device. The contacts can be adapted for use with devices having land grid array (LGA) and ball grid (BGA) I/O terminals.
2. Background Art
Sockets and connectors are necessary for separably interconnecting IC devices such as IC packages and electronic modules, to other devices, motherboards, test systems, and alike. A typical application is an electrical connector for connecting an IC device to the next level of electronic hardware or to a test unit. The contacts are positioned and maintained in a required array by an insulating housing which has contact receiving cavities, disposed in a pattern corresponding to the array of I/O terminals in each of the mated devices. The connector is interposed between the terminals of the devices and provides a separable interface to each device. The connector is typically attached by clamping to a PCB or a system motherboard. The clamping preloads each contact against a respective I/O terminal on the PCB. The other end of the contact extends from the housing and is adapted to connect to the corresponding I/O terminal of the mating device.
In electrical contacts, contact force and contact compliance (deflection capability) are important considerations. The contact force must assure a low contact interface resistance without being excessive. The contact compliance must account for planarity tolerances (z-directions variation of I/O terminal location due to tolerances, bowing of the board, etc.,), and to provide an adequate contact engagement even in worst case I/O terminal positioning. However, miniaturization of contacts often leads to contact force and compliance problems since the small contacts tend to be stiff and have a low deflection capability, while the manufacturing tolerances, assembly tolerances, and board planarity, do not scale accordingly and remain substantially the same as for larger contacts.
Many contacts and most test probes rely on a coil spring to provide the contact force and the resilient compliance necessary to assure that the contact force is in the desired range in the worst case tolerance conditions. Coil springs are relatively easy to manufacture in varying sizes, configurations, materials, and degree of compliance. In addition to providing a contact force, a compression spring may also serve as a conductive member. However, a coil spring acts as an electrical inductor at high frequencies and therefore presents electrical performance problems. Furthermore, it is often desirable to make the spring from a music wire or stainless steel which have poor electrical conductivity. Various mechanisms have been employed to mitigate the adverse electrical effects of the spring, as illustrated by the patents cited below.
A typical contact probe consists of a hollow barrel, a spring, and two plungers. The spring and the body portions of the plungers which are guided in the barrel, are retained in the barrel which is rolled or crimped at both ends. In order to reduce the contact resistance and inductance, the conventional contact probes rely on the plungers randomly tilting (i.e., deviating from axial alignment with the barrel) and electrically shorting to the barrel, to enable the current to bypass the spring and flow through the barrel. This conductive coupling significantly lowers the overall contact resistance of the probe and the parasitic electrical effects of the coil spring. The plunger-to-barrel contact depends on the spring bias, fit tolerances of the plunger and the barrel, contact surface topography, and plating uniformity on the inside of the barrel. The diametrical clearance between the plunger and the barrel must be precisely controlled to prevent an excessive tilt. The contact between the plunger and the bore of the barrel is localized along the line of a sliding contact between the edge of the plunger and the bore and an excessive tilt often causes accelerated wear of contacting surfaces. Since the surface of the bore is often irregular and plating of the bore surfaces can be inconsistent, the contact force between the plunger and the bore of the barrel is difficult to control. In severe cases, gauging of surfaces may expose base metal, cause oxidation of surfaces and an accumulation of a nonconductive debris between contact surfaces. This will cause a high contact interface resistance and/or high friction forces which can cause a plunger to seize in the bore.
A contact probe's cycle life is an important consideration in many test applications. If the probe length is short, the probe design, materials, and contact forces have significant impact on the cycle life of the probe and tradeoffs are necessary. The material of choice for miniature springs (e.g., having a mean coil diameter of less than 1.0 mm, and a wire diameter of about 0.1 mm, is music wire. Music wire has a very high tensile strength, and can provide a long mechanical service life at a high operating stress. However, music wire is made from a high carbon steel, is magnetic, and has low electrical conductivity. On the other hand, the preferred material for a spring that is used as a conductive member is beryllium copper, which has a higher conductivity but a lower elastic modulus and a lower strength than music wire.
Contacts have been proposed to address some of the above issues as illustrated by the following patents:
U.S. Pat. No. 7,535,241 (2009) to Sinclair discloses a contact having a barrel, a coil spring, and a plunger. The barrel has a stepped closed end which serves as a stop for the spring and allows the plunger body to conductively short to the barrel. This contact probe relies on a random tilting of the plunger for achieving a conductive contact with the inside surface of the barrel. The bottom of the barrel must be reliably plated, which is difficult, especially when small diameter, large aspect ratio barrels are used.
U.S. Pat. No. 5,990,697 (1999) to Kazama discloses a contact which utilizes a variable pitch coil spring as a primary conductive member. Such spring would be typically made from a higher conductivity alloy such as beryllium copper. The contact has some closely wound coils that become conductively shorted as the deflection progresses. Other coils must remain active so that a solid height is not reached. In order to satisfy the compliance requirement, these springs still require a substantial number of coils which are initially open, and only progressively are being closed (shorted) as the spring is being compressed. Such springs have a non-linear force vs deflection characteristics and can introduce a substantial variation in contact force and inductance due to manufacturing tolerances and non-planarity of mating interfaces. In worst tolerance cases, at a maximum deflection condition the contact force can be excessive, while at a minimum deflection condition an insufficient number of coils may be shorted so that the inductance can be excessively high.
U.S. Pat. No. 7,019,222 (2006) to Vinther discloses a one-piece coil spring contact wherein the coils are at an oblique angle to the direction of compression and are conductively shorted when the spring is compressed. While such contact can provide an excellent electrical performance, it is not scalable to smaller sizes without a significant loss of compliance. In this case, increasing compliance by increasing the number of coils will lead to a wider contact and will necessitate a larger contact-to-contact spacing. Furthermore, the contact is not easily adaptable for use with a variety of contact tips which are often needed to adapt the contacts to a particular I/O terminal configuration, such as a solder ball of a BGA device. In contrast, the conventional coil spring contacts are generally scalable to a smaller footprint by extending the spring length when the spring diameter is reduced. (Although this quickly leads to excessively long contacts with a high self-inductance.)
Other examples of low inductance contacts and probes can be found in U.S. Pat. Nos. 7,556,503 (2009) to Vinther; 7,134,920 (2006) to Ju et al; 6,696,850 (2004) to Sanders; 6,666,690 (2003) to Ishizuka et al; 6,043,666 (2000) to Kazama; 6,033,233 (2000) to Haseyama et al; and 5,641,314 (1997) to Swart et al.
The recent increases in circuit integration and operating frequencies pushed the available coil spring based contacts and probes to their performance limits. Consequently, there is a need for improved miniature contacts and probes having low contact inductance, low contact resistance, and which are suitable for use in sockets and connectors with close contact spacing and high contact count.